This project is the investigation of the cellular and molecular interactions during synaptogenesis. It was discovered that neuronal pre-synaptic cells which form functional cholinergic synapses with cultured muscle cells also produce a factor which causes the aggregation of acetylcholine receptors on muscle cells. The mechanism of this effect involves the lateral motion of acetylcholine receptors in the myotube membrane. The material responsible for the effect is a large soluble heat-labile macromolecule release into the culture medium by the presynaptic cells. There is pre-synaptic cellular specificity in the production of this molecule because many non-neuronal cells do not produce detectable amounts of the aggregation factor. The factor is species-non-specific, causing the aggregation of acetylcholine receptors on muscle cells of mouse, rat and chicken.